Eldest daughter syndrome is not a specific mental health diagnosis, but instead a way to describe some of the pressures and responsibilities placed on the firstborn daughter in a family, as per Kati Morton, LMFT, a licensed marriage and family therapist.
Firstborn daughters might experience "being responsible for more domestic labor and being held to a higher standard than their siblings," Morton told Health.
"Eldest daughters also share that they felt like the third parent, and are often responsible for the emotional caretaking of their entire family."
Simply because of their age, older siblings will often take on additional responsibilities for one, according to Julia Rohrer, Dr, rer, nat, who is a personality psychologist at the Wilhelm Wundt Institute for Psychology at Leipzig University in Germany.
"They can be assigned certain tasks earlier, help out [and] watch their siblings, and so on," she told Health.
As the division of labour is still oftentimes gendered, girls and women usually face extra pressures and work, Rohrer explained.
"In many families, women end up doing more chores and more care work," she said.
"If you combine these two things, there is a good chance that in at least some families, the eldest daughter will end up with more responsibilities than all of the other kids.”